1. Field of the Invention
The present invention concerns a blood velocity and acceleration measurement system. More particularly, it concerns an improved, non-invasive blood velocity and acceleration measurement device based on the use of Doppler ultrasound, and capable of providing measurements of peak velocity and peak acceleration of blood components, as well as a mean velocity and acceleration thereof. In addition, a method and apparatus for calibrating the blood flow measurement devices are disclosed.
2. Brief Discussion of Prior Art
Velocity and acceleration of blood flow in blood vessels are believed to be an important diagnostic tool. In particular, the peak acceleration of aortic blood flow has been recognized to be an excellent index of ventricular funcition. "A substantial body of evidence indicates that the peak acceleration of the blood corpuscles ejected by the left ventricle into the ascending aorta is the most sensitive indicator of ventricular performance" Rushmer, Cardiovascular Dynamics, p. 365 (4th ed. 1976). Rushmer's conclusion is that the acceleration of blood corpuscles in the aorta would be a valuable index of the influence on cardiac function of various perturbations, e.g., such as coronary occlusion, exercise, and drug infusion--a conclusion which Rushmer demonstrated experimentally. Other observers have considered peak acceleration, peak velocity, mean velocity, and mean acceleration of the blood corpuscles to be useful indices of cardiac performance.
There is a recognized need for a non-invasive indicator of ventricular performance that can provide ready access to reliable data regarding critical variables or parameters of cardiac performance. ("Invasive" techniques are those involving physical penetration of the body, such as by surgical opening of a portion of the body to permit insertion of a measurement device, or by injection of dyes to permit X-ray visualization.)
Recognized authorities have stated that it is not possible to measure aortic blood flow acceleration non-invasively. See Gams, Huntsman, and Chimoskey, Peak Aortic and Carotid Flow Acceleration in Trained Unanesthetized Dogs, Federation Proceedings, American Physiological Soc'y (1973). A prevalent technique presently used for evaluating cardiac performance in human patients is the radioisotope ventriculogram procedure. However, the necessary equipment is extremely expensive; specifically trained and licensed technicians are needed; and the tests involve insertion of a catheter into the patient's vein. Other invasive techniques are available in the case of animal experimentation, but the techniques are usually unsatisfactory for human patients. For example, some experimenters have inserted measuring devices into or adjacent to the aortas of laboratory dogs and humans. In the past, such invasive techniques have been used to strengthen medical understanding of blood flow. Although they are helpful for research purposes, such invasive techniques are usually not clinically practical or create risks to the patient.
Doppler ultrasound is used as a clinical and research tool in the evaluation of blood circulatory dynamics. The use of Doppler ultrasound may involve determining the speed of a reflecting material by beaming ultrasonic waves at the object and then measuring the frequency shift in the ultrasound waves reflected by the material.
Experimental work has indicated that ultrasound signals for blood velocity measurement can conveniently be transmitted into the body via the suprasternal notch, thereby facilitating evaluation of the ascending aortic or distal aortic arch blood flow. This "acoustic window" (as we may term the place via which ultrasound is beamed into the body) may be used in many types of patients.
Presently available electronic circuitry for decoding Doppler shift signals for blood flowmeter purpose generally use the "zero-crossing detector" method of determining Doppler shift frequencies. This is the method used, for example, in White U.S. Pat. No. 4,205,687. The method develops an RMS value of frequency which is not directly indicative of peak or mean velocity or acceleration. As shown by Lunt in his paper Accuracy and Limitations of the Ultrasonic Doppler Blood Velocimeter in Ultrasound in Medicine & Biology, 2:1-10 (1975), it is hard to achieve accurate blood velocity measurements with the zero-crossing detector method.
Techniques commonly in use for detection of peak Doppler frequencies present various problems. The fast Fourier transform ("FFT") technique requires expensive and elaborate equipment, and presently known FFT systems are not fast enough to measure peak acceleration. Other known peak frequency detection systems, such as voltage-controlled high pass filters, phase-lock loop systems, and double filters are noise sensitive, have a limited band width and frequency response, and are sensitive to amplitude modulation ("AM") of the signal.
Skidmore and Follett, in Ultrasound in Med. & Biol., 4:145 (1978), suggest Doppler-shift measurement of blood velocity. They suggest detection of maximum Doppler frequency by use of a voltage controlled high pass filter. But their system has not been put into commercial use. It is believed that the reason is that it is too sensitive to noise. Also, it appears not to be able to measure the frequencies associated with the highest velocity corpuscles, which are considered of greatest interest.
Callicot and Lunt, in "A maximum frequency detector for Doppler blood velocimeters", J. Med. Engineering & Technoloqy 3:80 (1979), note the use of phased lock loop techniques to measure peak velocity and mean velocity, and disclose a system in which a voltage controlled oscillator in a feedback loop is used to detect maximum frequencies representative of blood corpuscle velocities. Callicot et al does not show tracking of the high frequency edge of the frequency spectrum. FIG. 2 of the article illustrates this in that it shows that the system significantly underestimates the true peak velocity as measured by the sonagram. The slope of the velocity is not tracked. The system cannot respond rapidly enough to accurately detect peak acceleration. Moreover the system cannot measure peak velocity as shown by their illustrations. Neither Skidmore and Follett's or Callicot and Lunt's circuit has sufficient frequency response to accurately measure peak aortic acceleration.
There, thus, exists a need for a non-invasive technique for measurement of peak aortic acceleration. The need could be satisfied by a Doppler ultrasound device, if one could be devised (a) that was relatively inexpensive, noise-free, and insensitive to AM, and (b) that had a sufficient bandwidth and frequency response to register higher Doppler frequencies and rate of change of frequencies. It is believed by the inventor that the need for a non-invasive technique is well recognized in the art, and that other workers have sought to satisfy this need by a Doppler type device. The inventor does not believe that it is recognized in the art that the above-stated requirements must be met for a satisfactory Doppler ultrasound system. In any event, no such satisfactory Doppler system known to the inventor is presently available. The present invention concerns such a system: a real-time Doppler ultrasound, non-invasive system for measuring peak aortic (or other vessel) acceleration and velocity, which is relatively inexpensive, noise free, insensitive to AM, and with band width and frequency response sufficient to track high velocity, high acceleration blood component movement. Moreover, there is a need for non-invasive techniques for accurately and inexpensively determining mean blood velocity and acceleration.
These needs are realized by the techniques disclosed as follows.